Nitrate isotope anomalies reflect N2fixation in the Azores Front region (subtropical NE Atlantic)
The subtropical northeast Atlantic has previously been identified as a marine environment with an apparent imbalance between low nitrate supply to the surface and concurrent high export production. To better constrain the sources and fluxes of mixed layer nitrate and to assess the potential role of...
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ftoceanrep:oai:oceanrep.geomar.de:29718 2023-05-15T17:41:21+02:00 Nitrate isotope anomalies reflect N2fixation in the Azores Front region (subtropical NE Atlantic) Bourbonnais, Annie Lehmann, Moritz F. Waniek, Joanna J. Schulz-Bull, Detlef E. 2009 text https://oceanrep.geomar.de/id/eprint/29718/ https://oceanrep.geomar.de/id/eprint/29718/1/Bourbonnais.pdf https://doi.org/10.1029/2007JC004617 en eng AGU https://oceanrep.geomar.de/id/eprint/29718/1/Bourbonnais.pdf Bourbonnais, A., Lehmann, M. F., Waniek, J. J. and Schulz-Bull, D. E. (2009) Nitrate isotope anomalies reflect N2fixation in the Azores Front region (subtropical NE Atlantic). Journal of Geophysical Research: Oceans, 114 (C3). C03003. DOI 10.1029/2007JC004617 <https://doi.org/10.1029/2007JC004617>. doi:10.1029/2007JC004617 info:eu-repo/semantics/restrictedAccess Article PeerReviewed 2009 ftoceanrep https://doi.org/10.1029/2007JC004617 2023-04-07T15:20:44Z The subtropical northeast Atlantic has previously been identified as a marine environment with an apparent imbalance between low nitrate supply to the surface and concurrent high export production. To better constrain the sources and fluxes of mixed layer nitrate and to assess the potential role of N2 fixation in providing new nitrogen (N), we investigated the depth distribution of nitrate δ15N and δ18O at six stations across the Azores Front in the NE Atlantic. In addition, we measured the δ15N of dissolved organic N (DON) in surface waters and of sinking particulate N collected in sediment traps at 2000 m depth between 2003 and 2005 at Station KIEL276. The nitrate isotope profiles at the majority of the hydrographic stations displayed a decrease in the δ15N from depth toward low-nitrate surface waters, concomitant with an increase in δ18O. Given that nitrate uptake by phytoplankton leads to a proportional increase in nitrate δ15N and δ18O, the observed surface water nitrate isotope anomalies (Δ(15;18) up to −6‰) indicate that nitrate assimilation is not the sole process controlling the isotopic composition of nitrate in the photic zone and implicate a significant addition of newly fixed N that is remineralized in surface and subsurface waters. Both the concentration of DON and its δ15N in surface water were spatially invariant, showing mean values of 4.7 ± 0.5 μmol L−1 and 2.6 ± 0.4‰ (n = 35), respectively, supporting the conjecture of a mostly recalcitrant DON pool. The weighted biannual mean δ15N of sinking particulate N (1.8 ± 0.8‰, n = 33) was low with respect to thermocline nitrate. The anomalous dual nitrate isotope signatures together with the low δ15N of export production and elevated nitrate-to-phosphate ratios in surface and subsurface waters strongly suggest that N2 fixation represents a substantive source of N in this part of the subtropical northeast Atlantic. Simple isotope mass balance suggests that, locally, N2 fixation supplies between 56 and 259 mmol N m−2 a−1 for phytoplankton growth in the ... Article in Journal/Newspaper Northeast Atlantic OceanRep (GEOMAR Helmholtz Centre für Ocean Research Kiel) Journal of Geophysical Research 114 C3 |
institution |
Open Polar |
collection |
OceanRep (GEOMAR Helmholtz Centre für Ocean Research Kiel) |
op_collection_id |
ftoceanrep |
language |
English |
description |
The subtropical northeast Atlantic has previously been identified as a marine environment with an apparent imbalance between low nitrate supply to the surface and concurrent high export production. To better constrain the sources and fluxes of mixed layer nitrate and to assess the potential role of N2 fixation in providing new nitrogen (N), we investigated the depth distribution of nitrate δ15N and δ18O at six stations across the Azores Front in the NE Atlantic. In addition, we measured the δ15N of dissolved organic N (DON) in surface waters and of sinking particulate N collected in sediment traps at 2000 m depth between 2003 and 2005 at Station KIEL276. The nitrate isotope profiles at the majority of the hydrographic stations displayed a decrease in the δ15N from depth toward low-nitrate surface waters, concomitant with an increase in δ18O. Given that nitrate uptake by phytoplankton leads to a proportional increase in nitrate δ15N and δ18O, the observed surface water nitrate isotope anomalies (Δ(15;18) up to −6‰) indicate that nitrate assimilation is not the sole process controlling the isotopic composition of nitrate in the photic zone and implicate a significant addition of newly fixed N that is remineralized in surface and subsurface waters. Both the concentration of DON and its δ15N in surface water were spatially invariant, showing mean values of 4.7 ± 0.5 μmol L−1 and 2.6 ± 0.4‰ (n = 35), respectively, supporting the conjecture of a mostly recalcitrant DON pool. The weighted biannual mean δ15N of sinking particulate N (1.8 ± 0.8‰, n = 33) was low with respect to thermocline nitrate. The anomalous dual nitrate isotope signatures together with the low δ15N of export production and elevated nitrate-to-phosphate ratios in surface and subsurface waters strongly suggest that N2 fixation represents a substantive source of N in this part of the subtropical northeast Atlantic. Simple isotope mass balance suggests that, locally, N2 fixation supplies between 56 and 259 mmol N m−2 a−1 for phytoplankton growth in the ... |
format |
Article in Journal/Newspaper |
author |
Bourbonnais, Annie Lehmann, Moritz F. Waniek, Joanna J. Schulz-Bull, Detlef E. |
spellingShingle |
Bourbonnais, Annie Lehmann, Moritz F. Waniek, Joanna J. Schulz-Bull, Detlef E. Nitrate isotope anomalies reflect N2fixation in the Azores Front region (subtropical NE Atlantic) |
author_facet |
Bourbonnais, Annie Lehmann, Moritz F. Waniek, Joanna J. Schulz-Bull, Detlef E. |
author_sort |
Bourbonnais, Annie |
title |
Nitrate isotope anomalies reflect N2fixation in the Azores Front region (subtropical NE Atlantic) |
title_short |
Nitrate isotope anomalies reflect N2fixation in the Azores Front region (subtropical NE Atlantic) |
title_full |
Nitrate isotope anomalies reflect N2fixation in the Azores Front region (subtropical NE Atlantic) |
title_fullStr |
Nitrate isotope anomalies reflect N2fixation in the Azores Front region (subtropical NE Atlantic) |
title_full_unstemmed |
Nitrate isotope anomalies reflect N2fixation in the Azores Front region (subtropical NE Atlantic) |
title_sort |
nitrate isotope anomalies reflect n2fixation in the azores front region (subtropical ne atlantic) |
publisher |
AGU |
publishDate |
2009 |
url |
https://oceanrep.geomar.de/id/eprint/29718/ https://oceanrep.geomar.de/id/eprint/29718/1/Bourbonnais.pdf https://doi.org/10.1029/2007JC004617 |
genre |
Northeast Atlantic |
genre_facet |
Northeast Atlantic |
op_relation |
https://oceanrep.geomar.de/id/eprint/29718/1/Bourbonnais.pdf Bourbonnais, A., Lehmann, M. F., Waniek, J. J. and Schulz-Bull, D. E. (2009) Nitrate isotope anomalies reflect N2fixation in the Azores Front region (subtropical NE Atlantic). Journal of Geophysical Research: Oceans, 114 (C3). C03003. DOI 10.1029/2007JC004617 <https://doi.org/10.1029/2007JC004617>. doi:10.1029/2007JC004617 |
op_rights |
info:eu-repo/semantics/restrictedAccess |
op_doi |
https://doi.org/10.1029/2007JC004617 |
container_title |
Journal of Geophysical Research |
container_volume |
114 |
container_issue |
C3 |
_version_ |
1766142864437280768 |